TRPC channels are nonspecific cation channels widely expressed in human tissues (Montell 2005). They were the first found mammalian homologues of Drosophila TRP (Wes et al., 1995, Zhu et al., 1995). Seven proteins, referred to as TRPC1-7, constitute the canonical (or classical) TRP subfamily that is the closest related to the archetypal Drosophila TRP (30-40% identity, Okuhara et al., 2007).
Based on amino acid sequence homology, tissue distribution and functional similarities, TRPCs can be subclassified into four groups (Clapham et al., 2001; Montell, 2001). Being quite unique within the TRPC family, TRPC1 and TRPC2 each constitute a subfamily by themselves while TRPC4 and -5 are merged just as TRPC3, -6, and -7. The closely related TRPC3, TRPC6 and TRPC7 channels share a common activation mechanism and diacylglycerol (DAG) has been identified as their endogenous ligand (Hofmann et al., 1999; Okada et al., 1999).
According to the WHO, 30% of all deaths worldwide were caused by various cardiovascular diseases in 2005. Thus, there is an immense medical need for new medicines that prevent and treat cardiovascular diseases. TRPC channels are considered important pharmacological targets for the development of novel medicines for several cardiovascular pathologies including cardiomyopathy, vascular remodelling, hypertension and high endothelial permeability (Dietrich et al., 2007).
But so far, there are many open questions regarding their native composition and activation mechanisms, physiological functions, and roles in pathophysiology and disease. In situ identification of native TRPC channels is complicated by their wide and partially overlapping distribution, potential heteromultimerization, similar electrophysiological properties and a paucity of tool compounds to unequivocally trace these channels (Moran et al., 2004).
Known TRPC organic inhibitors such as 2-aminoethoxydiphenyl borate (2-APB) {Xu et al., 2005}, SKF 96365 (Boulay et al., 1997; Zhu et al., 1998) and BTP2 (He et al., 2005), and inorganic blockers such as Gd3+ and La3+ are not potent and specific enough (Li et al., 2005).
Studies in transgenic mice already have proven to be valuable in unravelling possible physiological functions of certain TRPCs (Freichel et al., 2001). But these model systems do not exist for each of the seven mammalian TRPC channels so far, their generation and analysis is time- and cost-consuming and they also have limitations as they are vulnerable to compensatory effects by closely related channels (Dietrich et al., 2005c).
Thus, the aim of the present invention was to develop a new pharmacological tool that allows to discriminate between and within the TRPC subfamilies, thereby allowing to elucidate the roles of the different channels under physiological as well as patho-physiological conditions.
According to the present invention, this is achieved with the use of norgestimate to selectively inhibit TRPC3, TRPC6 and TRPC7. Thus, norgestimate allows to pharmacologically distinguishe channels belonging to different TRPC subfamilies. In addition, norgestimate does not interfere with the common G protein-coupled receptor, Gq, phospholipase Cβ pathway that mediates TRPC channel activation in many cells. These properties make norgestimate a preferable tool for the identification and modulation of TRPC3, TRPC6 and TRPC7.